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Car Forum / MINI / February 2004Emissions Problems
Hi, I was enquiring if anyone could offer any advice to the problem I'm having getting a 1996 multi point fuel injection mini through its MOT. I originally passed the emissions test on the 1st test but it failed on an un-related problem (ball joint). I rectified the problem and retested a week later. However the garage could not get the car to pass the emissions test on 4 seperate attempts and had to fail the car. The diagnostic read out failed it on the Lambda reading at fast idle that should be bewteen (0.970 -1.030) however the reading for my car is 1.032 also the Natural idle test was CO: 0.55% which also failed. Is there any suggestions that could indicate the cause of the problem or will I have to run a full diagnostic on the engine management system? I'm extremely puzzled how it passed one week and only compled 15 miles between the re-test and failed on the 2nd occasion ? Thanks Andy > Hi, > [quoted text clipped - 18 lines] > > Andy If the re-test was at the same testing station, within 7 days of the original test then they should only have tested what it failed on first time round AFAIK. They shouldn't have even tested the emissions again if it was a retest and it passed the first time. check your air filter, check exhaust for any leak (however small), faulty coil (?). make sure car is upto proper temp (new 82C thermostat), check lead to temp sender has clean connection. most likely cause is a small exhaust blow, this fools lambda sensor and ecu runs engine slighty too rich. I had big probs getting good seal with stainless rc40 onto cat. chris > check your air filter, check exhaust for any leak (however small), > faulty coil (?). make sure car is upto proper temp (new 82C > thermostat), check lead to temp sender has clean connection. Make sure the engine really is up to temperature before testing. If the test station is only 3 miles away from home, take the long way there, especially in winter. > most likely cause is a small exhaust blow, this fools lambda sensor and > ecu runs engine slighty too rich. How does an exhaust leak change the composition of the exhaust gases? Does it draw fresh air in, introducing oxygen, forcing the mixture rich? Andy is getting a high lambda reading, indicating a rich mixture as detected by the oxygen sensor. If the mixture is in fact rich, it certainly isn't because the oxygen sensor is reading lean and pushing the system richer. > > most likely cause is a small exhaust blow, this fools lambda sensor and > > ecu runs engine slighty too rich. > > How does an exhaust leak change the composition of the exhaust gases? > Does it draw fresh air in, introducing oxygen, forcing the mixture rich? A blowing exhaust anywhere around the cat or before the backbox will make any injection car, especially those with closed loop emission systems (most cat equipped cars), run quite richer than they normally do. A lambda sensor is an oxygen sensor, it senses unburnt air (oxygen) in the exhaust. Fuel and air burn cleanest at a ratio of 14parts air/ 1part petrol from what i remember. It tries to keep to this ratio throughout the rev range. A cylinder will draw in a mixture of petrol and air and compress it. Then spark. Then bang. If not enough petrol is burnt then not all the air drawn into the cylinder with it will be burnt, leading to more unburnt air in the exhaust (oxygen). The lambda sensor will sense this and the ecu will enrich the mixture for the next burn. It does this all the time and includes revs and throttle position to help it decide the mixture required. Ideally there should be next to no oxygen in the exhaust, indicating the petrol is burning all the air in the cylinder (and vice versa). Air drawn in through an exhaust leak will fool the lamba sensor into thinking the mixture in the cylinder is too lean, the ecu reads this and richens up the mixture. Eventually a point is reached when the fuel will burn all the air in the cylinder. However the amount of air isn't now enough to burn all the fuel! This is why the car will now fail on Hydrocarbons (unburnt fuel) though it may pass on carbon dioxide. > Andy is getting a high lambda reading, indicating a rich mixture as > detected by the oxygen sensor. If the mixture is in fact rich, it > certainly isn't because the oxygen sensor is reading lean and pushing > the system richer. A lambda sensor does NOT detect fuel/air ratio! only air! The ecu uses this reading to help determine mixture! chris > Graham wrote: > > How does an exhaust leak change the composition of the exhaust gases? > > Does it draw fresh air in, introducing oxygen, forcing the mixture > > rich? > A blowing exhaust anywhere around the cat or before the backbox will > make any injection car, especially those with closed loop emission > systems (most cat equipped cars), run quite richer than they normally > do. A "blowing exhaust" which leaks exhaust out of the system does not change the composition of the gases reaching the oxygen sensor. If on the other hand fresh air, containing oxygen is drawn into the system at that point, it might. This is what I was asking about. > A lambda sensor is an oxygen sensor, it senses unburnt air (oxygen) Snip teaching grandma to suck eggs. > > Andy is getting a high lambda reading, indicating a rich mixture as > > detected by the oxygen sensor. If the mixture is in fact rich, it > > certainly isn't because the oxygen sensor is reading lean and pushing > > the system richer. > A lambda sensor does NOT detect fuel/air ratio! only air! Thats right. The stoichoimetric fuel:air ratio for LPG or methane is different to petrol, but in each case the oxygen sensor detects the presence or absence of oxygen in the exhaust, and hence whether the mixture is rich or lean or the stoichiometric mix. > The ecu uses this reading to help determine mixture! The oxygen sensor provides a signal of up to about 2 volts if there is no oxygen in the exhaust gases. A low voltage indicates oxygen present, a high voltage indicates oxygen absent. Andy is getting a high lambda reading, indicating a rich mixture *as detected by the oxygen sensor*. As far as the oxygen sensor is concerned, the mixture looks rich. So the mixture is not being pushed rich by an oxygen sensor which is reading lean because of fresh air leaking into the exhaust system between the motor and the sensor. Thanks for all the information I managed to get through the MOT by buying some injection cleaner and putting this in the tank and blasting out the rubbish from the system. I also managed to get hold of a nearly new CAT and replaced the current box with another one. > > Graham wrote: > > > How does an exhaust leak change the composition of the exhaust gases? [quoted text clipped - 40 lines] > reading lean because of fresh air leaking into the exhaust system > between the motor and the sensor. Thanks for that, The car was re-tested exactly 7 days after the original test and the MOT station assured me that they had to re-test emissions and the work completed for the re-test only. It's interesting that you mention the RC40 as I have recently had an RC40 fitted to the Cat. The MOT tester tried to seal the system while over the pit and he said the system did seem to be sealed and it was still showing over the set level. I'm reluctantly getting a diagnostic check tomorrow and this may show up any problems. If the recommendations are too costly for this minor problem, I will try the suggestions below. I have replaced the thermostat around 12 months ago but cleaning the temp sender and trying to check the exhaust system fully again may just get her through. The joys of technology on a simple car!! Andy > > Hi, > > [quoted text clipped - 36 lines] > > chris I thought on a retest they just checked what they failed it on. The Muffin Man > Hi, > [quoted text clipped - 18 lines] > > Andy > > Hi, > > [quoted text clipped - 20 lines] > > > > Andy Have a look on this link around page 17/18 http://www.via.gov.uk/mot/working_mot/pdfs/sn_04_2003.pdf You will see that an MPI Mini Lambda is in the range 0.95 - 1.09, so yours should have passed. The range 0.97 - 1.03 is for the new BMW MINI. Andrew > Have a look on this link around page 17/18 > http://www.via.gov.uk/mot/working_mot/pdfs/sn_04_2003.pdf > > You will see that an MPI Mini Lambda is in the range 0.95 - 1.09, so > yours should have passed. The range 0.97 - 1.03 is for the new BMW > MINI. That's terrible, I can't believe a testing station would be that dumb. Well no I suppose it is in the realm of 'the believable'.
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